1. Field of the Invention
The present invention relates to an index signal generating circuit and, more particularly, to an index signal generating circuit that generates an index signal responsive to the rotational condition of a spindle motor which is driven to rotate a rotating recording medium such as a floppy disk.
2. Description of the Related Art
Recording and reproducing apparatuses that record and reproduce information using a disk recording medium typically employ a spindle motor to rotate the disk recording medium, and along with it, an index signal generating circuit to generate an index signal indicative of the rotational condition of the spindle motor.
FIG. 10 is a block diagram showing an exemplary conventional index signal generating circuit, and FIG. 11 is a waveform diagram illustrating the principle of index signal generation by the index signal generating circuit shown in FIG. 10. Referring to FIG. 11, solid-line waveforms represent the ones obtained under a normal ambient temperature (i.e., about a room temperature of 20.degree. C.), and broken-line waveforms represent the ones obtained under a low ambient temperature.
As shown in FIG. 10, the index generating circuit comprises a single-pole magnet 102 (of a north pole or south pole) attached on the circumference of the rotor 101 of a spindle motor, a Hall effect device 103 arranged in the vicinity of the rotor 101, for detecting a change in magnetic flux from the single-pole magnet 102 when the rotor 101 rotates and a comparator 105, the first and second inputs of which are respectively connected to the Hall effect device 103 and a reference voltage power supply 104 to compare the detected output by the Hall effect device 103 with the reference voltage from the reference voltage power supply 104.
Referring to FIG. 11, the operation of the index signal generating circuit thus constructed is now discussed.
When the rotor 101 of the spindle motor is rotating, the single-pole magnet 102 also rotates integrally therewith. In the course of rotation, the single-pole magnet 102 approaches and then goes farther away from the Hall effect device 103, repeatedly. In its position apart from the single-pole magnet 102, the Hall effect device 103 is virtually free from the effect of magnetic flux of the single-pole magnet 102, and its detected output of is nearly zero. In its position near the single-pole magnet 102, the Hall effect device 103 is under the effect of magnetic flux of the single-pole magnet 102. In this way, the detected output of the Hall effect device varies depending on its distance to the single-pole magnet 102. The detected output V.sub.F from the Hall effect device 103 is obtained as shown by a curve a(V.sub.F) in FIG. 11, and fed to a first input of the comparator 105. Meanwhile, the comparator 105 receives, at its second input, a reference voltage Vref from the reference voltage power supply 104 to compare it with the detected output V.sub.F. FIG. 11 shows a comparison result V.sub.C as denoted by a curve b(V.sub.C). One edge, for example, the rising edge, of the comparison result V.sub.C is used to form an index signal.
In the conventional index signal generating circuit discussed above, the rotational condition of the spindle motor is detected by the Hall effect device 103, and the comparator 105 compares the detected output V.sub.F of the Hall effect device 103 with the reference voltage Vref of the reference voltage power supply 104, and the comparison output V.sub.C is used to form the index signal. Thus, the index signal generating circuit is of a relatively simple construction.
During the detection of magnetic flux, the sensitivity of the Hall effect device 103 used in the above index signal generating circuit varies with an ambient temperature change. When the ambient temperature drops below the normal room temperature, the magnetic flux detection sensitivity of the device 103 increases depending on the degree of temperature drop, and the detected output V.sub.FH output by the Hall effect device 103, represented by a curve c(V.sub.FH) in FIG. 11, becomes larger in magnitude than the detected output V.sub.F obtained under the normal room temperature. Front and back crossing points where the detected output meets the reference voltage are spaced more apart when the large detected output V.sub.FH is compared with the reference voltage Vref than when the normal magnitude detected output V.sub.F is compared with the reference voltage Vref. As a result, the comparison output V.sub.CH obtained under the low temperature as shown by a waveform d(V.sub.CH) in FIG. 11 has one edge (rising edge) shifted more frontward and the other edge (falling edge) shifted more backward.
Because of temperature characteristics unique to each Hall effect device 103, the above index signal generating circuit suffers shifting of the index signal timing with temperature rise and fall, and is unable to generate a precise index signal.